stroke_raster.h

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/*
    This file is part of Ansel,
    Copyright (C) 2026 Aurélien PIERRE.
 
    Ansel is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
 
    Ansel is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
    along with Ansel.  If not, see <http://www.gnu.org/licenses/>.
*/
 
#ifndef DT_WIDGETS_STROKE_RASTER_H
#define DT_WIDGETS_STROKE_RASTER_H
 
/* A stroke rasteriser for GUI overlays.
 *
 * An overlay line -- a mask's outline, its feather border -- arrives as a polyline already
 * sampled at the resolution it is shown at: one vertex per device pixel or so. Handing that to
 * cairo means turning pixels back into a vector path and asking a general-purpose stroker to
 * tessellate and rasterise it, twice per line (a wide dark pass under a narrow bright one). What
 * the overlays actually use of a stroker is small and fixed: two widths, two colours with alpha,
 * two dash patterns, round or butt caps, and a one-pixel antialiasing ramp. This is that, and
 * nothing more, painting straight into the ARGB32 pixels of the surface the overlay is drawn on.
 *
 * It knows nothing about masks, nodes or handles. Everything drawn with a cairo primitive that
 * is NOT a long polyline -- discs, arcs, arrows, the CLEAR-operator punch under a node -- stays
 * with cairo, on the same surface, before or after these strokes as the caller likes.
 *
 * The coverage model is a distance field: for every pixel within reach of a polyline, the
 * distance to the nearest point of it (a union of capsules, one per segment, so joins and caps
 * come out round with no seams). Coverage of a pass of half-width R is clamp(R + 1/2 - d, 0, 1),
 * the same one-pixel ramp CAIRO_ANTIALIAS_FAST draws. Dashes are cut along the polyline's arc
 * length before the capsules are stamped, so a dash end is a round cap exactly as cairo's
 * CAIRO_LINE_CAP_ROUND makes it. Compositing is premultiplied OVER, dark pass then bright pass,
 * which is what the two cairo_stroke() calls it replaces composed. */
 
#include <cairo.h>
#include <glib.h>
 
G_BEGIN_DECLS
 
typedef struct dt_stroke_pass_t
{
  double width;
  double red;
  double green;
  double blue;
  double alpha;
} dt_stroke_pass_t;
 
typedef struct dt_stroke_style_t
{
  dt_stroke_pass_t dark;     
  dt_stroke_pass_t bright;   
  double dash_on;            
  double dash_off;           
  gboolean round_caps;       
} dt_stroke_style_t;
 
gboolean dt_stroke_raster_can_paint(cairo_surface_t *surface);
 
gboolean dt_stroke_raster_polyline(cairo_surface_t *surface, const double *xy, int count,
                                   const dt_stroke_style_t *style);
 
gboolean dt_stroke_raster_path(cairo_t *cr, const dt_stroke_style_t *style);
 
gboolean dt_stroke_raster_touched(cairo_surface_t *surface, cairo_rectangle_int_t *touched);
 
void dt_stroke_raster_touched_reset(cairo_surface_t *surface);
 
G_END_DECLS
 
#endif // DT_WIDGETS_STROKE_RASTER_H
 
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